This invention relates to a device for detecting abnormality of a sensor system and, more particularly, to a device having two variable resistance sensors that are a main sensor, and a subsensor.
Hitherto, a control device having dual input means, which includes a main sensor and a subsensor, has employed the following methods so as to enable a decision on whether or not disconnection occurs in a sensor power supply system or in a sensor GND system. That is, such a control device has employed a method of supplying electric power to the main sensor and the subsensor from different two power supplies, or a method of supplying voltages of opposite polarities to the main sensor and the subsensor from a single common power supply by alternately being connected thereto, as described in the Unexamined Japanese Patent Application Publication Nos. Hei 4-83182 and Hei 4-214949.
FIG. 6 illustrates an example of a conventional control device configured so that electric power is supplied to a main sensor and a subsensor from two different power supplies. A sensor portion 1 has two variable resistance sensors that are a main sensor 11a and a subsensor 11b. A power supply line 13a for the main sensor 11a is connected to a sensor power supply of a calculating portion 2 through a connecting line 21a. A GND line 14a of the main sensor 11a is connected to a sensor GND 24a of the calculating portion 2 through a connecting line 22a. Further, a power supply line 13b for the subsensor 11b is connected to a sensor power supply 23b of the calculating portion 2 through a connecting line 21b. A GND line 14b for the subsensor 11b is connected to a sensor GND 24b of the calculating portion 2 through a connecting line 22b. Moreover, the main sensor 11a and the subsensor 11b are mechanically adapted by a joining member 10 to change in the same direction.
A main-sensor output 12a and a subsensor output 12b are inputted to a processing part 25 of the calculating portion 2. Further, the processing part 25 is operative. to decide whether or not anomalies, such as a disconnection and a short circuit, occur in a sensor system having the main sensor 11a and the subsensor 11b. Further, when it is decided that no anomalies occur in the sensor system, the processing part 25 follows a predetermined procedure and calculates a control variable converted into a digital value and drives a control portion 3. Conversely, when an anomaly occurs in the sensor system, the processing part 25 sets the control variable at a predetermined favorable value for fail safe operation, and drives the control portion 3.
FIG. 7 illustrates characteristics of the main-sensor output 12a and the subsensor output 12b. Incidentally, even in the case that disconnection occurs in the power supply system line (13a, 21a or 23a) or in the GND system line (14a, 22a or 23a) for the main sensor 11a, the subsensor output 12b is normally outputted. Therefore, a failure determination can be performed by comparing the value of the main-sensor output 12a with that of the subsensor output 12b. Additionally, in the case that the power supply system line and the GND system line are used in common between the main sensor and the subsensor, both the main sensor and the subsensor are at the same potential level when a break occurs in, for example, the power supply system line. Thus, there is a fear that such an electric potential level cannot be discriminated from a usual electric potential level. Consequently, the control device requires two line systems for each of the sensors.
However, in the case of the method of supplying electric power to the main sensor 11a and the subsensor 11b from two different power supplies, there is the need for a dual hardware system, that is, two sensor power supplies 23a and 23b and two sensor GNDs 24a and 24b of the calculating portion 2, and two connecting lines (21a, 22a, 21b, 22b) between the calculating portion and the sensor portion.
Further, in the case of the method of supplying voltages of opposite polarities to the main sensor and the subsensor from a single common power supply by alternately being connected thereto, the main-sensor output characteristics are the inverse of the subsensor output characteristics. Thus, when sensor output values are used, there is the necessity for a contrivance to, for example, reverse the output value of one of the sensors.
This invention is accomplished to solve the aforementioned problems. Accordingly, an object of the present invention is to provide a device for detecting abnormality of a sensor, by which the hardware configuration thereof can be simplified, and in which main-sensor output characteristics are the same as subsensor output characteristics.
According to a first aspect of the present invention, there is provided a device for detecting abnormality of a sensor system, which have two sensors that are a main sensor and a subsensor that operate in the same direction, characterized by comprising voltage application means for applying voltages of the same polarity to both of the main sensor and the subsensor from a common power supply, offset means for causing an output value of the subsensor to deviate from an output value of the main sensor by a predetermined value, and abnormality decision means for deciding, when a difference between the main sensor and the subsensor is outside a predetermined range, that an anomaly occurs in the sensor system.
In the case of a first embodiment of the device according to the first aspect of the present invention, the offset means of the device according to claim 1 comprises a resistor, which has a predetermined resistance value, between the sensor and a sensor power supply or between the sensor and a sensor GND.
In the case of a second embodiment of the device according to the first aspect of the present invention, the offset means of the first embodiment of the device according to the first aspect of the present invention comprises resistors of the same resistance value, which are respectively interposed between the main sensor and the sensor power supply and between the subsensor and the sensor GND.
According to a second aspect of the present invention, there is provided a control device for a vehicle, which comprises the device for detecting abnormality of a sensor according to the first aspect of the present invention, and a control portion, wherein the control portion is driven by calculating, when it is decided that no anomaly occurs in the sensor system, a control variable by following a predetermined procedure, and wherein the control portion is driven by setting, when it is decided that an anomaly occurs in the sensor system, the control variable at a favorable predetermined value for fail safe operation.